By way of background, a procedure known as “pigging” is well-established in the fields of oil and gas recovery and distribution. This involves driving a “pig”, being a solid body or device shaped to fit the pipeline interior, along the pipe under pressure. The pig may have scrapers or brushes to shift contaminants from the pipeline's interior surface (U.S. Pat. No. 5,457,841 and U.S. Pat. No. 5,903,945); additionally or alternatively it may be adapted to absorb or otherwise take up debris in the pipeline (U.S. Pat. No. 4,216,026). It is known to make a pipeline pig wholly or partly from an elastically deformable material which can pass bends or occasional irregularities such as welding joints in the pipeline without damage, while contacting the pipe surface firmly (see U.S. Pat. No. 4,389,461 and U.S. Pat. No. 5,924,158). It has been proposed to use a solid bullet of ice as a pig, since it will disintegrate spontaneously and harmlessly if it gets stuck. See e.g. U.S. Pat. No. 4,898,197 and U.S. Pat. No. 4,724,007. It has also been proposed to pass a self-sustaining gelled mass, made by gelling a hydrocarbon, along the pipeline e.g. to separate one product flow from another, or to keep the pipe dry when not in use.
Another established use of pigs is in the cleaning of heat exchanger tubes which suffer from the accumulation of internal residues from manufacture or during use: see U.S. Pat. No. 4,860,821.
In most piping systems however it is normal for cleaning to use direct methods such as brushing and/or flushing using cleaning liquids, which are easier to control and cheaper than pigs. Also, pigs cannot be used where fluid flow conduits are of substantially varying cross-section or include internal obstructions, whereas liquids can.